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Thread: Solar voltage?

  1. #1
    LHBA Member edkemper's Avatar
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    Solar voltage?

    What is the best or the most efficient voltage to use in a new off-rid system? For the moment, forget the difference in price of the batteries.

    In setting up a new system, this is a choice I can't find much info on.
    edkemper

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  2. #2
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    I am going with 48 volts.

    The usual options are 12, 24, and 48; 36 and 72 volts are hardly ever mentioned. The new Tesla batteries are much, much higher voltages.

    By far, the 12, 24, and 48 volt inverters are the easiest to find, and competitive on price. Those other choices are possible, but get pricey, with no real benefit.

    You asked about efficiency. For a given power being stored or delivered (volts times current), the higher voltage of the 48 V bank means lower current, and less loss in the cables. Those losses vary with the square of the current, so the loss goes up fast with current increase.

    But if you are careful to keep your cables short, it's not that big of a deal. I just would not run DC at 12 volts from one building to another, and would probably hesitate to run it across a room unnecessarily. Of course, it's still DC from the panels, so there is only so much care you can take.

    My short answer is 48 V.

  3. #3
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    For what it is worth, I'd also add that there is no difference in battery price, as far as the voltage choice goes. That is because batteries are not sold as 24, 48, or those other multiples.

    Lead acid batteries are made from 2 volt cells, including car batteries, etc. So for a 24 volt battery bank, for example, the two 12 volt batteries are wired in series, adding the values. Batteries can also be bought as 2 volt cells, as many as you want. They come as 6 volt versions, too, which really contain 3 cells inside. You just keep adding them up until you get the bank voltage you choose.

    Batteries are a whole other topic from your thread, but I just want to clarify that, for lead acid, there is no real trade off to consider or ignore when choosing bank voltage.

  4. #4
    LHBA Member rreidnauer's Avatar
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    48v is the way I'm going too. Much easier getting inverters and other equipment to work with it.
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  5. #5
    LHBA Member edkemper's Avatar
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    48v sounds like a good choice.

    I was also asking because of a special a battery company is offering. $12,000 for 36 2v with inverters and 4 chargers.

    I can do the math. But I was wondering what the company is selling. How do they expect them to be wired together at what rate?

    Thanks for the help.
    edkemper

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  6. #6
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    I have seen a charge controller that can handle 72 volts (36 of those 2 V).

    I have seen a charge controller that can handle 36 volts. For that special, 18 cells wired in series would give the 36 volts, and the term for it is a "string". You'd then wire two of those strings in parallel to use all 36 cells for a 36 volt bank.

    You could also have 3 parallel strings of 12 cells. Each string would have 24 volts, which brings us back to widely available inverters.

    I'm not sure which math you referred to, so please forgive me if I missed your point.

    I don't see a way to get a 48 volt bank out of those 36 cells, though.

    Something else that matters is the amp-hour rating of the cells. When you do that math, you get a good idea of the energy that the bank will deliver when fully discharged. From my electric bill, my average daily energy usage is about 8 kilowatt-hours, in the Spring and Fall (so not counting heating and air).

    It's crucial to know what the company is selling. For example, the chargers should be MPPT (maximum power point tracking), meaning smart enough to magically squeeze the most energy from your panels at one voltage and store it in the batteries at another voltage. If not MPPT, they are likely not a good deal at all. And if American made, all the better. It's not just about being patriotic -- there is a real reputation in this industry that Chinese equipment is rated "optimistically".

    For my charge controllers, arriving Wednesday (woo! hoo!), I first researched and then searched the 'net for the best street price on MidNite Solar's Classic 250. Then I wanted to do better than that. Sure enough, following the principle of patience taught by the LHBA, a deal turned up. MidNite Solar has a "garage sale" -- equipment they can't sell as new for whatever reason, but still looks and works fine, complete with warranty. I scored that for well below street price.

    :-)

    And the sale is still going on now. It's not advertised, just mentioned in the online forum. You have to know to ask.
    Last edited by donjuedo; 01-11-2016 at 02:43 PM.

  7. #7
    LHBA Member edkemper's Avatar
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    Doing the math didn't work out to 48. But the number of batteries offered made me wonder which voltage I should design for. BTW, the batteries are Trojan L16RE-2V 1110 AH 2-Volt Deep Cycle Batteries. The batteries appear to be a good choice 24 batteries being 49v and they have 26,640 AH total.

    Then again, I still have no idea how big a system we'll need so I'm only in the research stage.
    edkemper

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  8. #8
    LHBA Member rreidnauer's Avatar
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    Quote Originally Posted by edkemper View Post
    Doing the math didn't work out to 48. But the number of batteries offered made me wonder which voltage I should design for. BTW, the batteries are Trojan L16RE-2V 1110 AH 2-Volt Deep Cycle Batteries. The batteries appear to be a good choice 24 batteries being 49v and they have 26,640 AH total.

    Then again, I still have no idea how big a system we'll need so I'm only in the research stage.
    You didn't say whether the batteries and gear are new or used. 36 of those batteries can be bought new for $13k. You didn't specify inverter or charger voltages either, so I can't really make many suggestions on configuration options.

    You AH total is incorrect. One battery, or all 36 in series, the AH remains the same. What changes is voltage, and more importantly, wattage. You will need to get your mind into "wattage mode" since that is how capacity is really determined.

    Volt amps = watts
    Watts amps = volts
    Watts volts = amps
    Last edited by rreidnauer; 01-12-2016 at 03:28 AM.
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  9. #9
    LHBA Member edkemper's Avatar
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    Quote Originally Posted by rreidnauer View Post
    You will need to get your mind into "wattage mode" since that is how capacity is really determined.

    Volt amps = watts
    Watts amps = volts
    Watts volts = amps
    Again, the reason I came here to learn. Gracias. All part of the pre-build research.
    edkemper

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  10. #10
    While I have NO real world, hands on experience with them, on paper I rather like NiFe batteries. They're spendy and have poor power to weight ratio compared to lead acid batteries (and they cost more than 3 times as much but I like that they're far more tolerant of mistakes/abuse and that they have an obnoxiously long service life.

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